1. Field of the Invention
This invention relates to the field of chemical vapor deposition (CVD) onto a semiconductor substrate, and more particularly plasma enhanced chemical vapor deposition (PECVD) which utilizes a radio frequency (rf) field to supply energy to gaseous reactants.
2. Prior Art
In manufacturing semiconductor devices, it is often necessary to deposit a thin film on a semiconductor substrate. The purpose of the film in the completed semiconductor can vary from electrical conduction, insulation or for its semiconductor properties. The chemical composition of the film generally determines its function. Of concern in this application is the deposition of an amorphous dielectric, silicon nitride, or silicon oxynitride, onto a semiconductor substrate through use of PECVD.
PECVD utilizes an rf field to supply energy to gases within a reaction chamber or tube. The gases become excited and form glow discharge or plasma; plasma being defined as a partially ionized gas, and glow discharge being a plasma maintained over a specified pressure range (0.5 Torr-2 Torr). The plasma in turn transfers energy into reactant gases, also within the reaction chamber, to enhance the deposition of a thin film onto the substrate or layer formed on the substrate. For example, if silane (SiH.sub.4), nitrous oxide (N.sub.2 O) and ammonia (NH.sub.3) are used as reactant gases, a thin film of silicon nitride (Si.sub.x O.sub.y N.sub.z) is deposited on the substrate. The major advantage of PECVD is that the substrate need not be heated to high temperatures for deposition to occur and it is therefore useful, for example, to prevent deterioration of materials, unwanted diffusion of dopants, etc. In addition, PECVD produces films with desirable properties such as strong adhesion and the ability to protect the substrate from corrosive agents.
However, PECVD deposited films often incur internal tensile or compressive stress which can cause cracking and peeling of the film or damage to the substrate (or overlying layers formed on the substrate). In the deposition of silicon oxynitride, the internal tension could destroy the film's passivation and protection properties. The mechanism which causes the film stress is not fully understood, but it is speculated to be caused by the incorporation of reaction by-products into the film such as hydrogen, nitrogen, and oxygen. Additionally, the difference in contraction coefficients between the underlying layers and the silicon nitride layer when cooling can cause stress. For higher temperature depositions, the inclusion of by-products into the film is less pronounced, but the advantage of using PECVD at low temperatures (i.e., low temperature processing) is lost.
Consequently, other means to control stress in films deposited through PECVD have been sought. It is known that films deposited at radio frequencies generally below 1 MHz exhibit compressive stress while those deposited at frequencies above 1 MHz exhibit tensile stress. It has been found that less stress in the film develops if dual frequency radio waves are used to generate the rf field in the reaction chamber. As discussed in Abstract No. 385 of The Electrochemical Society, volume 86-2, page 580, one such radio wave is generated at a frequency below 500 KHz and the other at a frequency above 4 MHz. These frequencies are generated simultaneously and continuously over a predetermined deposition period. In this instance, the power of the lower frequency radio wave is controlled and a film with a lower compressive stress than conventionally deposited films is obtained. The frequencies are generated below 500 KHz and above 4 MHZ because the interval between these frequencies corrsponds to the International Communications window and cannot be used without employing costly shielding.
In a related dual frequency method described in Industry News, "Controlling Stress in PECVD Silicon Nitride", page 15, March, 1988, the high and low frequencies are alternated over a deposition period such that silicon nitride films are deposited in alternating layers; one layer exhibiting tensile stress and the layer(s) adjacent exhibiting compressive stress. The composite film, therefore, does not favor compressive or tensile properties which helps to relax the stress in the film as a whole.
The presently invented process also employs the use of dual frequencies to control stress and allows for the deposition of a thin film with lower stress than possible by employing conventional methods. However, instead of generating the dual frequency radio waves in an alternating fashion or continuously over the deposition period, as in the prior art, the invented method strikes the gases in the reaction chamber with a low frequency, high voltage radio wave to enhance plasma formation and then triggers a lower voltage high frequency radio wave. The high frequency wave is capable of causing further gas ionization and hence plasma formation. This method offers several advantages over the prior art dual frequency methods especially for batch PECVD systems such as the ASM.RTM. model plasma IIIA systems which can deposit up to 160 four-inch wafers at a time. (ASM.RTM. is a registered trademark of Advanced Semiconductor Materials America, Inc.)